Brandon Savage talks about the "cardinal sin" of working with object inheritance in PHP applications - adding public methods to a class that extends/implements another.

I know I've committed this sin, and you probably have too. The sin of which I speak is a grave one, and it violates several well known and established principles of object oriented application development. What is this sin of which I speak? It is none other than the addition of new public methods to an object that extends or implements abstract class or application interface, in violation of both the Liskov Substitution Principle and the Dependency Inversion Principle.

He talks some about the Liskov Substitution Principle first, pointing out that adding those new methods makes the new object non-replaceable as the Liskov principle requires. As far as the Dependency Inversion Principle, the practice breaks it because you'd be depending on those new methods as concrete, not abstracted from the parent. He makes a few recommendations as far as ways to prevent violating these principles including using multiple interfaces or creating multiple abstract classes for different public APIs.